It is not uncommon to find yourself needing to splice several small wires together and put heat-shrink tubing around them for insulation.
Many times, you need to splice some wires that are short. If you cut your heat-shrink tubing long enough to cover the splice you may find that - when placed on the wire, you can't get one end very far from the splice that needs to be soldered.
I find this happens when I want to shorten and connect several ESC power-feed wires together. I cut the heat-shrink tubing 1" or so long, and slide it over the wire. But the wire itself is only 1.5" long. Now, when I solder the wire, the heat flows through the heavy gauge copper wire and heats the heatshrink tubing to the point where it shrinks.
So what is the solution? Get a pair of medical hemostats. You might need some larger ones for the larger gauge wire, but clamp the wire with the hemostats between where you are soldering and the end of the heat-shrink tubing. The hemostats will act as a heatsink and prevent the tubing from shrinking before you want it to.